1. Field of the Invention
The present invention relates to a multi-stage sprocket assembly for a bicycle, the assembly including a small sprocket, a large sprocket disposed coaxially with the small sprocket and a shift assist projection provided to the large sprocket for assisting a shifting movement of a chain from the small sprocket to the large sprocket.
2. Description of the Related Art
A multi-stage sprocket assembly of the above-described type is known from e.g. the U.S. Pat. Ser. No. 4,348,200. In this assembly, two of the teeth of the large sprocket are cut away and the shift assist projection is integrally formed at this cutaway portion. In the operation of shifting the chain from the small sprocket to the large sprocket, the chain first comes into engagement with the shift assist projection and then with a tooth of the large sprocket, thus reducing an amount of `over-shift` of the chain. As a result, the chain shift can be effected by a relatively small manual force.
One problem of the above convention is that the chain tends to engage too deeply with the shift assist projection. This excessive engagement results in a large radial displacement of the engaged chain portion relative to the entire chain segment being shifted to the large sprocket and results consequently in locking of the chain portion at the projection thereby intefering with the chain shift.
Further, according to this conventional sprocket assembly, the distance between a tooth of the large sprocket which first engages with the shifting chain portion (i.e. a tooth adjacent the cutaway portion) and the shift assist projection is not set substantially as an integer multiple of a disposing pitch of the large sprocket teeth. Because of this, the shifting chain portion tends to ride on a tooth crest of the large sprocket, thus intefering with the chain shift.